In a report, Moody's also cut its outlook for South Korea's economic growth for next year to 2.1 percent from 2.2 percent.

"The weaker global economy has stunted Asian exports, and the uncertain operating environment has weighed on investment," Moody's said in the report. "In particular, softer capital formation has mirrored the weakening in exports, especially for trade-reliant economies such as Korea and Hong Kong."

South Korea's exports have been on a steady decline since December last year, again falling 13.3 percent in the first 20 days of August from the same period last year.

Last month, South Korea's central bank downgraded its growth outlook for this year to 2.2 percent from a 2.5 percent forecast three months earlier.